AN EVENING WITHOUT LIZA MINNELLI -- At the New Playwrights' Theater through December 31.

Liza Minnelli is not the only thing missing from the New Playwrights' "An Evening Without Liza Minnelli." Half the evening is missing, too.

The absence of Minnelli isn't crucial, because of the presence of two quick comic actors, Jan Frederick Shiffman and Dana Vance, who performs a variety of deft characterizations in ten short sketches. But Tim Grundmann, who has written so many delightful musicals for the New Playwrights' Theater, has short-changed this one.

The show is less than an hour long, and if the actors hadn't pointedly said goodbye at the end of it, the audience would have gone out fro a ten-minute stretch and showed up again. Many of the skits are cute ideas but not thoroughly worked out, and some are not cute ideas, which were not examined with a critical eye and then dropped. All together, they are simply not thick enough, in length or content, to add up to theater, although there is lots of good salvageable nightclub material.

And that's where a Minnelli would have helped. She would presumably know the difference between nightclub material, for an audience also drinking and socializing, and theatrical material, for people who are lined up on straight chairs paying close attention.